feat: backup, notifier, organizer, douban, watch-history, poster-wall

Complete the port of the remaining MediaStation features:

Backend services:
  - service/backup.go: SQLite hot backup via VACUUM INTO, list, delete,
    restore. Backup files stored under {data_dir}/backups/ with timestamps.
  - service/notifier.go: Multi-channel push notifications (Telegram /
    Bark / WeChat Server酱 / Webhook). Configuration via settings table
    keys notify.telegram.*, notify.bark.*, etc.
  - service/organizer.go: Auto-rename + move media files into library
    directory structure. Movie: {Title} ({Year})/{Title} ({Year}).ext,
    TV: {Title}/Season XX/{Title} - SxxExx.ext. Both per-media and
    per-library batch endpoints.
  - service/douban.go: Douban (豆瓣) metadata provider using the
    unofficial subject_suggest API. Returns Chinese titles + posters.
    Requires douban_cookie in secrets config.

Handlers:
  - handler/backup.go: GET/POST/DELETE /admin/backups, POST restore.
  - handler/organizer.go: POST /admin/media/:id/organize,
    POST /admin/libraries/:id/organize.
  - handler/notify.go: POST /admin/notify/test.

Frontend pages:
  - WatchHistoryPage.tsx: full paginated history with progress bars,
    resume buttons, poster thumbnails and timestamps.
  - PosterWallPage.tsx: dense 8-column poster grid of all media
    (matches the original MediaStation PosterWallView.vue).

Sidebar additions: 观看历史 + 海报墙 links in the main nav group.

Verified: go build, go vet, go test all pass; tsc -b && vite build
emits 30 lazy chunks; main bundle 254 KB / 85 KB gzipped.

internal/handler/backup.go

@@ -0,0 +1,52 @@
+// Package handler — database backup/restore endpoints.
+package handler
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"github.com/ShukeBta/MediaStationGo/internal/service"
+)
+
+func createBackupHandler(svc *service.Container) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		info, err := svc.Backup.Create(c.Request.Context())
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+			return
+		}
+		c.JSON(http.StatusOK, info)
+	}
+}
+
+func listBackupsHandler(svc *service.Container) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		items, err := svc.Backup.List()
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{"items": items})
+	}
+}
+
+func deleteBackupHandler(svc *service.Container) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if err := svc.Backup.Delete(c.Query("filename")); err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+			return
+		}
+		c.Status(http.StatusNoContent)
+	}
+}
+
+func restoreBackupHandler(svc *service.Container) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if err := svc.Backup.Restore(c.Request.Context(), c.Query("filename")); err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{"message": "restored — restart the server to apply"})
+	}
+}